It is known in the metal die casting industry to employ a gas evacuation system of the axial melt impingement type.
The system includes a valve movable to an open position where the die or mold cavity is in communication with a vacuum pump through a vacuum conduit or to a closed position where the mold cavity is not in such communication. As the gas evacuation system is engaged on the mold during normal operation after mold warm-up, the valve is moved to the open position by application of pneumatic pressure to a cylinder in which an extension on the valve is slidably received as a piston. After positioning in the open position, the valve is held in the open position mechanically by engagement of spring biased balls against an intermediate stem portion of the valve. Pneumatic pressure in the cylinder is discontinued once the valve is so engaged and held in open position. A molten charge is then injected into the mold cavity with gas in the cavity being evacuated from the mold cavity past the open valve. The valve is closed by impingement of the molten charge on the valve head after gas is substantially evacuated from the cavity.
Occasionally, the valve does not open properly prior to injection of the melt in the cavity and, no gas is evacuated from the mold cavity even though the vacuum pump has been actuated. A die casting having increased porosity may result in the event molten metal is injected into a mold cavity from which gas has not been evacuated.
There is a need for the machine operator to be able to determine quickly and easily if the gas evacuation system is operating properly to evacuate the mold cavity.